Phase-Dependent Passive Muscle Stretching Modulates B-Mode Ultrasound Classification of Parkinson's Disease.

OBJECTIVE: Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by tremor, rigidity and bradykinesia. Although central nervous system pathology has been extensively studied, peripheral musculoskeletal manifestations in PD remain comparatively understudied. METHODS: In this exploratory study, the characteristics of the gastrocnemius muscle were assessed using B-mode ultrasound in 28 PD patients and 18 age-matched controls. Dynamic B-mode ultrasound videos were acquired during passive stretching induced by ankle rotation using a continuous passive motion device, while shear wave elastography was recorded concurrently to quantify muscle stiffness. The stretching sequence was divided into three phases-initial (low elasticity), transitional (increasing elasticity) and final (high elasticity)-with phase boundaries standardized using the shear wave elastography-derived stiffness progression. For classification, only B-mode images were used as model input, and a VGG19 model was trained separately for each phase to discriminate PD from the controls. RESULTS: Classification performance increased progressively across the phases, with accuracy improving from 52% in the initial phase to 73% in the final phase, indicating that PD-related muscle features become more discriminative under higher tension. CONCLUSION: PD muscles exhibit phase-dependent differences from healthy controls that are detectable on B-mode ultrasound. These results support the potential of phase-resolved peripheral musculoskeletal imaging during passive stretching as a complementary biomarker for PD-related neurodegenerative processes.